Production of hyper-pure monocrystal-line rods in continuous operation



y 9, 96 SIEBERTZ 3,036,892

K. PRODUCTION OF HYPER-PUREMONOCRYSTALLINE RODS IN CONTINUOUS OPERATION Filed March 4, 1959 States My invention relates to a method of producing highly purified monocrystalline semiconductor rods. It particularly relates to an improvement in processes, known particularly for silicon, according to which a monocrystal of a substance is segregated or drawn from a body of the same substance, which body is in liquid condition at the region of its surface at which the monocry-stal is being withdrawn from it.

More particularly, the invention concerns a method in which the semiconductor material is present as a component or components of a highly-pure gaseous compound thereof. The semiconductor is precipitated from the gaseous compound on the surface of a drop-shaped melt consisting of the same highly purified semiconductor material. The term highly purified semiconductor material is meant to denote a semiconductor material whose concentration of lattice defection atoms is below the concentration of degeneration. Such a semiconductor material is often designated in industry by such terms as electronic-grade silicon or electronic-grade germanium. The conductivity of the silicon or germanium rods made according to the method of the present invention amounts particularly to less than 1 ohm" .cm. preferably at room temperature (20 C.). The germanium is purified up to the condition of intrinsic conductance. In the case of silicon its specific resistance is above ohmcm.

It is an object of my invention to devise a crystal pulling method of the above-mentioned type that is capable of economically producing a monocrystalline semiconductor rod of any desired length in continuous operation. A significant feature of the process is the use of a monocrystalline rod-shaped crystal germ of small cross section which is immersed in the adjacent surface of the melt in order to pull a thin monocrystalline semiconductor rod by subsequently removing the crystal germ from the melt, the cross section of the crystal germ being small in comparison with the melt surface facing the germ, viz. preferably about one half to less than one tenth of the melt surface area. The pulling speed is so chosen that approximately just as much semiconductor material solidifies on the thin monocrystalline rod as is newly formed by thermal disassociation of the highly pure compound at the surface of the melting zone. This condition need not be satisfied at any moment. The operation may even be such that a noticeable difference occurs temporarily between the silicon quantities being pulled out of the melt and those that are precipitated into the melt. However, it is decisive that the quantity of the thin rod pulled in one pass of operation is considerably larger than the quantity of the melt at the beginning of the operation. This requirement is secured by the above-mentioned condition, so that, for example, the pulling proper may be performed intermittently and the precipitation of the semiconductor material may take place not during the pulling but only during the intermediate intervals.

In the continuously performed process of segregation and monocrystal production, the thin monocrystal rod is pulled from a melt located at the end of a thick rod, and the melt is held in position by its surface tension and preferably additionally by a levitating or supporting electromagnetic field. The process is started with a relatively 3,h3fi,892 Patented May 29, 1962 very thick semiconductor rod whose diameter is preferably at least about 2 cm. or larger. At the beginning of the method a thin semiconductor germ penetrates into the melt of the thick rod, the germ representing ultimately the end of the thin monocrystal rod pulled out of the melt.

At the point of contact between the two rods the semiconductor material of the thick rod is melted, for example by high frequency, or by an arc, so that the melt rests on top of the thick rod approximately in hemispherical shape with the monocrystal germ immersed in the molten zone. This assembly is located in an atmosphere which contains a gaseous or vaporous compound of the semiconductor material, for example silicon tetrachloride and hydrogen. Then, the semiconductor material is continuously segregated or precipitated from this compound by thermal decomposition at the surface of the molten zone. This process is to be so conducted that the segregation takes place predominantly within the molten zone. The production of silicon, for example, by thermal decomposition of the silicon compound and precipitation of the silicon onto a melt is already known as such. My improvement comprises a process in which a monocrystal germ, or thin monoorystalline rod is caused to penetrate into the melting zone, and is to be pulled out of the melt preferably with a speed such that the quantity of the semiconductor material being precipitated is at least approximately equal to the quantity of the semiconductor material which is brought to crystallize on the crystal germ. This condition, as explained above, is to be substantially satisfied during one pass of operation. This makes it possible to continuously pull a thin monocrystal rod from a spatially stationary calotte or cap of molten material, the thin rod containing considerably more material than the calotte. The pulled monocrystal rod is preferably kept in revolution during the pulling operation. In some cases it is also prefer-able to turn the lower portion of the assembly including the thick rod, the turning being in the same sense or the opposite sense, and using a correspondingly chosen speed of rotation.

According to the invention the ratio of the diameters of the thick and thin semiconductor rods respectively is preferably made so large or such that the solidifying front in the pulled thin rod is practically planar. When the solidifying front extends in a plane penpendicula-r to the axis, disturbances of the crystal formation, for exam ple dislocations or the like, are considerably reduced because internal thermally caused tensions in the rod are reduced to a great extent. When penorming the method, the melting zone can be supported by an electromagnetic field, whereby the surface of the molten semiconductor material can be increased so that a relatively large surface has the desired decomposition temperature and the segregated quantity of the semiconductor material is also increased. The operating conditions described in this paragraph give a measure for the thickness that can be given to the monocrystal rod to be pulled out of the melt.

Once the method has been placed into operation, the thin rod can be pulled out of the reaction vessel in accordance with the growth of its length, through a seal in the vessel wall. This has the advantage that the guiding means for the monocrystal rod can be mounted outside of the reaction space proper and that this portion of the thin monocrystal rod can be cooled for reducing the stress and wear imposed upon the synthetic, organic, or non-metallic, components of the equipment. By suitable admixtures to the reaction gas or in any other known manner, for example by introducing foreign substances, including doping agents, into the melt, any desired resistance or conductance characteristic of the crystal can 3 be simultaneously adjusted and can be varied continuously or discontinuously.

The invention will be further explained with reference to the accompanying drawing describing an embodiment exemplary of processing apparatus employed according to the invention.

A reaction vessel 1 is provided with a gas supply duct 9 and a gas discharge duct 8. Mounted in the vessel is a rod 2 of Semiconductor material, viz. silicon, usually of polycrystalline constitution. Located at the upper end of the rod 2 is the melting zone 4 in which the lower end of a monocrystal germ 5 is immersed. The gas mixture continuously introduced through conduit 8, and containing a gaseous semiconductor compound, is continuously decomposed at the melt and replenishes it with semiconductor substance. The thin monocrystal rod 3 is pulled out of the melt. Any desired doping substances are admixed to the gaseous mixture supplied through conduit 9. However, the doping substances may also be admixed to the reaction gas intermittently, so that p-n junctions are formed in the monocrystalline rod 3 during the pulling operation.

The heating and supporting field assembly 6 comprises a heating-wire winding 61, and a supporting i.e. levitating electromagnetic field coil 62 traversed by high-frequency current. However it is also advantageous to use a highfrequency coil for heating as well as for supporting, which by virtue of its location and by proper choice of the high frequency, produces the melt and causes a radial pressure to be exerted by the coil field upon the molten material, thus preventing the material from dropping oif. The induction heating coil and/ or supporting field coil may also be mounted outside of the reaction vessel. In this case the vessel is given a smaller cross section so that it surrounds the semiconductor rod assembly more closely than illustrated. The technique of a supporting field results in the production of cylindrical rods which are so smooth that they can be passed through a sealing sleeve 12, which is only schematically illustrated on the drawing, or if desired can pass through several sealing sleeves between which pre-vacuum chambers are located.

By virtue of the fact that the mechanical guiding means for the rod being pulled are located outside of the reaction space proper, a relatively very simple design of a continuously operating drive can be used. The drive may comprise two roller pairs 10 and 11 of a synthetic material of suitable elasticity which engage the semiconductor rod and which place the crystal rod in rotation while simultaneously advancing it out of the reaction vessel. If the axis of the roller pair 11 is not placed parallel to the axis of the crystal rod but is slightly inclined relative thereto, then the upward advancing motion of the rod can be obtained without using the roller pair 10. The advancing motion is adapted, to maximum extent practically feasible, to the rate of precipitation of the semiconductor material in the reaction zone in order to permit conducting the melting process continuously without frequent readjustment. Where the advancing speed is small in comparison with the speed of rotation, the axes of the drive rollers 11 for imparting rotation to the rod are not or need not be inclined. In such case the advancing motion is effected by the axially active rollers 10. For reducing the thermal stresses imposed upon the synthetic, organic, or non-metallic, components of the equipment, for example the sleeve 12 and the rollers 10, 11, a cooling device '7 is provided between the reaction vessel 1 and the organic components. The device 7 may be a cooling pipe disposed about and spaced from the rod 3. Since the gas mixture from which the semiconductor material is segregated is continuously replenished and the segregated semiconductor material, when converted into part of the monocrystal, is continuously removed, the method according to the invention aflords or results in a continuous production of thin semiconductor rods of any desired length,

from which the desired wafers of semiconductor material can be cut off at a point beyond the mechanical guiding elements 10 and 11 of the apparatus.

When a silicon monocrystal is to be made, the rod 2 is polycrystalline silicon, for example. Silicon of high purity is precipitated thereon by reduction, or by thermal decomposition or dissociation, of suitable, preferably prepurified, silicon compounds. Suitable starting compounds are the silicon halogenides, particularly the silicon chlorides, which may be caused to react with a reducing agent or other reactive substance, such as hydrogen, to produce silicon. Preferably, rod 2 is of the highest feasible purity. For example, where the rod 2 is of silicon, a gas mixture of hydrogen and silicon tetrachloride vapor or of silicon hydrogen tetrachloride vapor is introduced at 9. The expedients used for such introduction described in the application of Schweickert et al., Serial No. 736,387, filed May 19, 1958, can be used here. The tetrachloride-hydrogen mixture is already reactive at 1100 to 1200 C.

When germanium or other material is to be precipitated, the silicon rod 2 can be replaced by rods of germanium or said other material. To produce monocrystalline germanium of high purity, germanium tetrachloride (GeCl and hydrogen as carrier gas and reducing agent, can be employed. They are already reactive in the range between 700 and 800 C. Other examples of semiconductors are found in said prior application Serial No. 736,387, and also in Schweickert et al. application Serial No. 665,036, filed June 11, 1957, the disclosures of which are incorporated herein by reference.

In the much preferred process, the supporting piece or block 2, of silicon, germanium, etc. is underneath the molten drop or globule 5. However, it is within the purview of the process to turn or invert the apparatus degrees, or any other angle.

In the preferred process the piece 2 is supported in position, with or without rotation, and the rod 3 is pulled upwardly from it. However, it is within the scope of the invention to displace pieces 2 and 3 with respect to each other, by moving either or both pieces 2 and 3 upwardly and/ or downwardly, during the pulling operation.

As mentioned in the introductory part of the text above, the melt has an approximately hemispherical shape and rests upon the thick rod. For a rod of a given diameter and the known density of silicon or germanium, the resulting weight of the molten drop is readily calculated.

I claim:

1. A crucible-free method of producing a silicon monocrystalline rod comprising melting the upper tip only of a first silicon rod to form a molten globule of silicon supported on the solid main body of the first silicon rod, and contacting with said globule a vapor substance comprising a compound of silicon which substance yields silicon on contact with the molten globule, said vapor substance being supplied to the space zone about said globule at substantially the rate at which it yields silicon to the globule, contacting a seed crystal of monocrystalline silicon with said globule, said seed crystal having a smaller cross-sectional area than the area of the superface of the globule, and pulling the seed crystal upwardly from the molten globule to form a monocrystalline rod of smaller cross section than the first silicon rod, the quantity of material of the mono crystalline rod pulled in any one pass of the crystal pulling operation being greater than the quantity of material comprising the globule at any one moment of the operation, the pulling speed being such that approximately as much silicon solidifiers on the monocrystalline silicon rod being pulled as is newly formed by thermal dissociation of said vapor substance on the globule.

2. A crucible-free method of producing a silicon monocrystalline rod comprising melting the upper tip only of a piece of silicon to form a molten globule of silicon supported on the solid main body of the piece of silicon, and contacting with said globule a vapor sub tance comprising a halide of silicon which substance yields silicon on contact with the molten globule, contacting a seed crystal of monocrystalline silicon with said globule, said seed crystal having a smaller crosssectional area than the area of the superface of the globule, and pulling the seed crystal upwardly from the molten globule to form a monocrystalline rod of smaller cross section than the first silicon rod, the quantity of material of the monocrystalline rod pulled in any one pass of the crystal pulling operation being greater than the quantity of material comprising the globule at any one moment of the operation.

3. A crucible-free method of producing a germanium monocrystalline rod comprising melting the upper tip only of a first germanium rod to from a molten globule of germanium supported on the solid main body of the first germanium rod, and contacting with said globule a vapor substance comprising a compound of germanium which substance yields germanium on contact with the molten globule, contacting said globule with a seed crystal of monocrystalline germanium, said seed crystal having a smaller cross-sectional area than the area of the superface of the globule, and pulling the seed crystal upwardly from the molten globule to form a monocrystalline rod of smaller cross section than the first germanium rod, the quantity of material of the monocrystalline rod pulled in any one pass of operation being greater than the quantity of material comprising the globule at any one moment of the operation.

4. A crucible-free method of producing a rod of monocrystalline material comprising melting the upper tip only of a polycrystalline piece of the same material to form a molten globule thereof supported on the solid main body of the polycrystalline piece, and contacting with said globule a vapor substance comprising a compound of said material which substance yields said material on contact with the molten globule, said vapor substance being supplied to the space zone about said globule at the rate at which it yields said material to the globule, contacting said globule with a seed crystal of said monocrystalline material, and pulling the seed crystal upwardly from the molten globule to form a monocrystalline rod of smaller cross section than the polycrystalline piece, the quantity of'material of the monocrystalline rod pulled in any one pass of the crystal pulling operation being greater than the quantity of material comprising the globule, the pulling speed being such that approximately as much material solidifies on the monocrysta'lline rod being pulled as is newly formed by thermal dissociation of said vapor substance on the globule.

5. The method of claim 1 in which the molten globule is at least partly supported by electromagnetic field levitation.

6. In a crucible-free method for producing a highly purified monocrystalline semiconductor rod in which the semiconductor material is precipitated by thermal decomposition from a highly pure gaseous compound of the material onto the surface of a molten globule of the same highly pure semiconductor material, the improvement characterized in that a semiconductor monocrystalline seed crystal of said material is contacted with the melt, the cross-sectional area of the seed crystal being less than the surface area of the melt facing the seed crystal, thereafter a monocrystalline semi conductor rod thinner in cross-sectional area than the area of said surface area is pulled by relatively displacing the seed crystal with respect to the globule, the displacing pulling speed being such that approximately just as much semiconductor material solidifies on the thin monocrystalline rod as is being concomitantly formed on the surface of the molten globule by thermal decomposition of the gaseous highly pure compound, the pulling being carried out in a sealed reaction space, the monocrystalline rod being withdrawn therefrom, as it is pulled, in sealed relation to said space, and being cooled prior to being operatively seized for carrying out said pulling, the pulled rod being rotated.

7. In a crucible-free method for producing a highly purified monocrystalline semiconductor rod in which the semiconductor material is precipitated by thermal decomposition from a highly pure gaseous compound of the material onto the surface of a molten globule of the same highly pure semiconductor material, the improvement characterized in that a semiconductor monocrystalline seed crystal of said material is contacted with the melt, the cross-sectional area of the seed crystal being less than the surface area of the melt facing the seed crystal, thereafter a monocrystalline semiconductor rod thinner in cross-sectional area than the area of said surface area is pulled by relatively displacing the seed crystal with respect to the globule, the displacing pulling speed being such that approximately just as much semiconductor material solidifies on the thin mono-crystalline rod as is being concomitantly formed on the surface of the molten globule by thermal decomposition of the gaseous highly pure compound, the molten globule being supported on the upper end of a piece of the semiconductor, which piece is fixed in position, the monocrystalline rod being pulled upwardly in a substantially vertical direction, the monocrystalline rod and the seed crystal each having crosssectional areas sufficiently less than the said surface area of the molten globule so that the solidifying front at the lower end region of the latter rod is substantially planar and is transverse to the lengthwise axis of the monocrystalline rod.

8. The process of claim 7, the material being silicon, the gaseous compound being a halogenide of silicon.

References Cited in the file of this patent UNITED STATES PATENTS 2,631,356 Sparks et al Mar. 17, 1953 2,851,342 Bradshaw et al. Sept. 9, 1958 2,892,739 Rusler June 30, 1959 FOREIGN PATENTS 1,125,277 France July 9, 1956 OTHER REFERENCES Nelson: Article in Transistors 1, RCA Laboratories, pages 66-76, March 1956. 

1. A CRUCIBLE-FREE METHOD OF PRODUCING A SILICON MONOCRYSTALLINE ROD COMPRISING MELTING THE UPPER TIP ONLY OF A FIRST SILICON ROD TO FORM A MOLTEN GLOBULE OF SILICON SUPPORTED ON THE SOLID MAIN BODY OF THE FIRST SILICON ROD, AND CONTACTING WITH SAID GLOBULE A VAPOR SUBSTANCE COMPRISING A COMPOUND OF SILICON WHICH SUBSTANCE YIELDS SILICON ON CONTACT WITH THE MOLTEN GLOBULE, SAID VAPOR SUBSTANCE BEING SUPPLIED TO THE SPACE ZONE ABOUT SAID GLOBULE AT SUBSTANTIALLY THE RATE AT WHICH IT YIELDS SILICON TO THE GLOBULE, CONTACTING A SEED CRYSTAL OF MONOCRYSTALLINE SILICON WITH SAID GLOBULE, SAID SEED CRYSTAL HAVING A SMALLER CROSS-SECTIONAL AREA THAN THE AREA OF THE SUPERFACE OF THE GLOBULE, AND PULLING THE SEED CRYSTAL UPWARDLY FROM THE MOLTEN GLOBULE TO FORM A MONOCRYSTALLINE ROD OF SMALLER CROSS SECTION THAN THE FIRST SILICON ROD, THE QUANTITY OF MATERIAL OF THE MONOCRYSTALLINE ROD PULLED IN ANY ONE PASS OF THE CRYSTAL PULLING OPERATION BEING GREATER THAN THE QUANTITY OF MATERIAL COMPRISING THE GLOBULE AT ANY ONE MOMENT OF THE OPERATION, THE PULLING SPEED BEING SUCH THAT APPROXIMATELY AS MUCH SILICON SOLIDIFIERS ON THE MONOCRYSTALLINE SILICON ROD BEING PULLED AS IS NEWLY FORMED BY THERMAL DISSOCIATION OF SAID VAPOR SUBSTANCE ON THE GLOBULE. 